空類，虛函數類，虛繼承類的空間大小

 

//此代碼在32位win下運行成功
#include<iostream>
using namespace std;
class A//A是空類，編譯器會用一個char類型標記這個類，大小爲1
{

};
class B:public A//B繼承了A，但同樣也是空類，大小爲1
{

};
class C:virtual public A//虛擬繼承的時候編譯器會安插一個指向父類的指針，大小爲4
{

};
class D//大小爲4
{
  public:
  int a;
  static int b;//靜態變量被放在靜態存儲區
};
class E //print函數不佔內存空間，大小爲4
{
    public:
    void print(){cout<<"E"<<endl;}
    private:
    int a;
};
class F//虛函數會佔用一個指針大小的內存，系統需要用這個指針維護虛函數表。大小爲8
{
    public:
    virtual void print(){cout<<"F"<<endl;}
    private:
    int a;
};
class G:public F//多了一個虛函數內存大小不變，可見一個類只有一個虛函數指針。大小爲8
{
    public:
    virtual void print(){cout<<"G"<<endl;}
    virtual void print2(){cout<<"G2"<<endl;}
};
int main()
{
    A a;
    B b;
    C c;
    D d;
    E e;
    F f;
    G g;
    cout<<sizeof(A)<<" "<<sizeof(a)<<endl;//1 1
    cout<<sizeof(B)<<" "<<sizeof(b)<<endl;//1 1
    cout<<sizeof(C)<<" "<<sizeof(c)<<endl;//4 4
    cout<<sizeof(D)<<" "<<sizeof(d)<<endl;//4 4
    cout<<sizeof(E)<<" "<<sizeof(e)<<endl;//4 4
    cout<<sizeof(F)<<" "<<sizeof(f)<<endl;// 8 8
    cout<<sizeof(G)<<" "<<sizeof(g)<<endl;//8 8

    return 0;
}